Natural gas is predicted to outlast oil reserves by a significant margin and large quantities of methane, the main component of natural gas, are available in many areas of the world. Natural gas often contains about 80-100 mole percent methane, the balance being primarily heavier alkanes such as ethane. Alkanes of increasing carbon number are normally present in decreasing amounts in crude natural gas streams. Carbon dioxide, nitrogen, and other gases may also be present. Most natural gas is situated in areas that are geographically remote from population and industrial centers making it difficult to utilize these gas resources. The costs and hazards associated with the compression, transportation, and storage of natural gas make its' use economically unattractive. Also, in some regions where natural gas is found combined with liquid hydrocarbons, the natural gas is often flared to recover the liquids. This wasted resource also contributes to global carbon dioxide emissions and to undesirable global warming.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water (also called steam reforming) to produce carbon monoxide and hydrogen (i.e., synthesis gas or “syngas”). The reaction is shown in equation 1:CH4+H2O=>CO+3H2(ΔH°298=206.1 kJ/mol),  Equation 1                methane-steam reforming.        
In a second step, the syngas is converted to hydrocarbons. For example, Sasol Ltd. of South Africa utilizes the Fischer-Tropsch process to provide fuels that boil in the middle distillate range. Middle distillates are defined as organic compounds that are produced between kerosene and lubricating oil fractions in the refining processes. These include light fuel oils and diesel fuel as well as hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock is also a two stage process. In the first process methane is converted to carbon monoxide and hydrogen (syngas) by either steam reforming (see Equation 1) or by dry reforming. In the dry reforming process, carbon dioxide and methane are subjected to high temperature (generally between about 700 degrees C. to about 800 degrees C.) in the presence of a catalyst. This in turn forms hydrogen and carbon monoxide (see Equation 5). Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas.
During syngas synthesis, other reactions, such as a water gas shift reaction, occur simultaneously with reactions shown in Equation 1. One such water gas reaction is shown in Equation 2 and is frequently in a dynamic equilibrium state.CO+H2OCO2+H2  Equation 2
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. Syngas, once produced, can then be converted to other compounds useful in the chemical industries. The two step process, syngas formation followed by reforming reactions, such as methanol synthesis, requires two reactor stages and is inherently inefficient due to heat and material losses as well as the need for additional capital equipment for processing and separating the resulting gas and liquid streams. Such a process is disclosed in U.S. Pat. No. 6,797,851 to Martens et al., where two reactors are utilized to produce olefins with each reactor having a different catalyst.
A third stage has been practiced, also by converting the methanol produced into hydrocarbons composed of alkenes, alkanes, naphthas and aromatic compounds. The product distribution that is produced depends on the catalyst and the process conditions used for conversion of the methanol. Other more complex processes to convert natural gas to liquids have been described involving synthesis, transportation of the end product to another site followed by further processing (see U.S. Pat. No. 6,632,971 to Brown et al. which describes a process for converting natural gas to higher value products using a methanol refinery remote from the natural gas source).
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to its' significant inherent advantages, such as the significant heat that is released during the process, in contrast to steam reforming processes that consume large amounts of energy.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperatures and pressures. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 3.CH4+½O2=>CO+2H2  Equation 3
The partial oxidation reaction is exothermic, while the steam reforming reaction is strongly endothermic. The highly exothermic reactions of partial oxidation have made it inherently difficult to control the reaction temperature in the catalyst bed. This is particularly true when scaling up the reaction from a micro reactor (i.e., ¼ in (about 6 mm) diameter reactor tube and less than 1 gram of catalyst) to a larger scale commercial reactor unit because of the additional heat generated in large reactors and the limited heat transfer available in a larger reactor. If heat is not removed or controlled in such a way that temperature control can be maintained, partial oxidation can transition to full oxidation, with the major quantities of end products being relatively low value carbon dioxide and water. Furthermore, the oxidation reactions are typically much faster than the reforming reactions. The selectivity of catalytic partial oxidation to various end products are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. There is much prior art focusing on the partial oxidation of methane to syngas that then requires conversion to more valuable higher carbon number organic compounds in a second reaction stage. Many of the catalysts used in the prior art for the partial oxidation of methane have included precious metals and/or rare earth compounds. The large volumes of expensive catalysts needed by prior art for catalytic partial oxidation processes and the need for a separate reforming operation have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities (“GHSV”), and selectivity of the process to the desired products. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. An approach to prevent partial oxidation reactions of methane from creating primarily carbon dioxide and water is to limit the availability of oxygen in the reaction zone. This often, however, results in coke formation on the catalyst. Accordingly, substantial effort has been devoted in the art to develop catalysts allowing commercial performance without coke formation.
A number of processes have been described in the art for the production of either syngas and/or organic compounds with carbon numbers of 2 or more (also denoted as C2+ compounds) from methane via catalyzed partial oxidation reactions or the so called shift gas process followed by recombination of the syngas to produce organic compounds with carbon numbers of 2 or more.
As used herein, the term “C2+ compounds” refers to compounds such as, but not limited to, ethylene, ethane, propylene, butane, butene, heptane, hexane, heptene, octene and all other linear and cyclical hydrocarbons where two or more carbons are present. For the purpose of chemical analysis in the examples contained herein, organic compounds that remain in gaseous state were analyzed by means of gas chromatography and higher carbon number materials were collected as condensate liquids. Generally, gaseous materials have carbon numbers less than about 8.
The noble metals have been used as catalysts for the partial oxidation of methane, but they are scarce and expensive. Less expensive catalysts such as nickel-based catalysts have the disadvantage of promoting coke formation on the catalyst during the reaction, which results in loss of catalytic activity. Metal carbides and nitrides have also been shown to exhibit catalytic properties similar to the precious metals. A. P. E. York et al., (Stud. Surf. Sci. Catal. (1997), 110 (3rd World Congress on Oxidation Catalysis, 1997), 711-720.) and Claridge et al. (J. Catalysis 180:85-100 (1998)) disclose the use of molybdenum and tungsten carbides as catalysts for the partial oxidation of methane to syngas but suffered from rapid catalyst deactivation.
U.S. Pat. No. 4,522,708 (Leclercq et al.) describes a process for reforming petroleum products by the catalysis of dehydrocyclization, isomerization, hydrogenolysis and dehydrogenation reactions, the improvement wherein the catalysts employed comprise a metal carbide.
U.S. Pat. No. 5,336,825 (Choudhary et al.) describes an integrated two step process for conversion of methane to liquid hydrocarbons of gasoline range.
U.S. Pat. No. 6,090,992 (Wu et al.) describes a carburized transition metal-alumina compound employed as a catalyst in the isomerization of a hydrocarbon feedstock comprising saturated hydrocarbons.
U.S. Pat. No. 6,207,609 (Gao et al.) describes a metastable molybdenum carbide catalyst for use as a catalyst for methane dry reforming reaction.
U.S. Pat. No. 6,461,539 to Gaffney describes metal carbide catalysts and a process for producing synthesis gas using a mixed metal carbide catalyst.
U.S. Pat. No. 6,488,907 (Barnes et al.) describes a method of converting a reactant gas mixture comprising hydrocarbon compounds with carbon numbers from 1 to 5 and oxygen into a product gas mixture comprising H2 and CO using a catalyst comprising a catalytically active component selected from the group consisting of rhodium, platinum, ruthenium, iridium, rhenium, and combinations thereof, supported on a catalyst support chosen from the group consisting of oxide-dispersion-strengthened alloys comprising aluminum, chromium, and yttrium oxide, at least one metal selected from the group consisting of iron, nickel, and cobalt, and, optionally, titanium, and non-oxide-dispersion-strengthened alloys comprising chromium, aluminum, titanium, an element selected from the group consisting of yttrium, lanthanum and scandium, and at least one metal selected from the group consisting of iron, nickel and cobalt, the catalyst having a metal oxide layer disposed between said catalytically active component and the support.
U.S. Pat. No. 6,518,476 (Culp et al.) describes methods for manufacturing olefins such as ethylene and propylene from lower alkanes, that is, methane, ethane and/or propane, by oxidative dehydrogenation at elevated pressure.
U.S. Pat. No. 6,555,721 (Griffiths et al.) describes a process for producing a mono-olefin from a feedstock containing a paraffinic hydrocarbon comprising feeding a gaseous paraffinic hydrocarbon-containing feedstock and a molecular oxygen-containing gas to an autothermal cracker wherein they are reacted in the presence of a catalyst.
U.S. Pat. No. 6,596,912 (Lunsford et al.) discloses processes and systems for the conversion of methane in high yields to C4+ hydrocarbons. The principal steps of the recycle process include reacting methane and O2 in an oxidative coupling reactor over a Mn/Na2 WO4/SiO2 catalyst at 800 degrees C. to convert the methane to ethylene, and oligomerizing the ethylene product by reacting it with an H-ZSM-5 zeolite catalyst at 275 degrees C. in a catalytic reactor for subsequent conversion of the ethylene to higher hydrocarbons.
U.S. Pat. No. 6,602,920 (Hall et al.) discloses a process for converting natural gas to a liquid by converting a fraction of the gas stream to reactive hydrocarbons, primarily ethylene or acetylene, and reacting methane and the reactive hydrocarbons in the presence of an acidic catalyst to produce a liquid, predominantly naphtha or gasoline.
U.S. Pat. No. 6,623,720 (Thompson et al.) discloses transition metal carbides, nitrides and borides, and their oxygen containing analogs useful as water gas shift catalysts.
U.S. Pat. No. 6,852,303 (Seegopaul et al.) discloses a molybdenum carbide compound for use as a catalyst for the methane dry reforming reaction and the water gas shift reaction.
U.S. Pat. No. 6,887,455 (Carpenter et al.) describes a reactor that utilizes a catalyst comprising rhodium dispersed on a refractory oxide support material which comprises as cations cerium and zirconium, wherein the weight ratio of cerium to zirconium in the support material is from 50:50 to 99.5:0.5. The catalyst is used in the self-sustaining combination of exothermic partial oxidation and endothermic steam-reforming to produce a gas-stream containing mainly hydrogen, carbon dioxide and nitrogen.
U.S. Pat. No. 6,930,068 (Kaneko et al.) describes a methanol reforming catalyst for generating hydrogen by reforming methanol in the atmosphere containing oxygen and steam contains a metal oxide support and Pd—Zr alloy. The reforming catalyst accelerate a steam reforming reaction of the methanol as an endothermic reaction and a partial oxidation reaction of the methanol as an exothermic reaction while suppressing generation of CO gas.
U.S. Pat. No. 7,186,670 B2 (Mamedov et al.) discloses the use of oxidation catalysts used either by themselves or in series with other oxidation catalysts to form benzene, ethylene and syngas.
United States Patent Application Pub. No. 2006/0155157 A1 (Zarrinpashne et al.) describes a catalyst for direct conversion of methane to ethane and ethylene. The example given utilizes 0.5 grams of catalyst in a micro-reactor configuration.
In a co-pending patent application (U.S. Ser. No. 11/517,839 to Bagherzadeh et al.) we disclosed a catalyst that demonstrates both exothermic reactions (oxidative coupling) and endothermic reactions (reforming), leading to the production of hydrocarbons having carbon numbers of 2 or greater from a feedstock such as methane gas. The catalysts used a halogen, such as chlorine, to maintain catalytic activity. Halogens can be corrosive, and can be difficult to handle at elevated temperatures. Carbon dioxide was used in the feed stream, and it is difficult to remove and/or recycle back into the feed stream.
The prior art describes i) the use of a capital intensive multi step Fisher-Tropsch process (syngas formation followed by reforming) to produce higher carbon compounds; ii) limitations in the size of reactors and amount of catalyst used due to the need to rapidly extract heat to avoid the formation of undesirable combustion products (primarily CO2 and H2O). Thermal instability will result if this technology were scaled up to commercial size; iii) processes utilizing corrosive halogens to promote and/or maintain catalyst activity. Other inventions have poor catalyst life and/or low conversions and yields of desired reaction products. Prior inventions have relied mainly on partial oxidation of methane that results in high levels of undesirable carbon oxides and water or dehydrogenation type mechanisms that result in carbon formation and coking of the catalyst.
The prior art does not contemplate the present invention that utilizes a unique combination of catalyst ingredients and preparation process to obtain high conversions of methane and high selectivity to C2+ organic compounds without the deficiencies of prior art.
Thus there is a continuing need for better processes and catalysts for the conversion of methane directly to higher carbon number organic compounds that can be directly used in chemical synthesis without going through the costly and inefficient step of first converting methane to syngas. The process and catalyst should exhibit long catalytic activity at high space velocities and be scalable to a size that can be utilized in a commercial process.